In the disc recording art, it is common to use read/write heads which react against the air moved with the rotating disc, causing the heads to "fly" a small distance from the disc surface. In the manufacture of such read/write heads, it is common to test the hydrodynamic characteristics of the heads so that the flying height characteristics are known, thereby avoiding the use of heads which fly too high or too low in relationship to disc surface and also avoiding heads which fly at an improper angle to the disc. Too high a flying height will result in lower areal density, while too low a flying height can cause head/disc interface failure.
Flying height testing is generally accomplished by means of a flying height tester, using optical interference techniques. Such a flying height tester comprises, for example, a monochromatic light source directing monochromatic light at a glass disc. The glass disc is rotated at speeds simulating the rotation of a magnetic disc, and the head assembly being tested is positioned in a holder in flying relation to the glass disc. Monochromatic light is directed at the disc at a predetermined angle to the surface thereof. Light is reflected from the surface of the disc closest to the flying head, as well as from the surface of the flying head itself, and impinges onto a light sensitive sensor.
The flying height between the head and the disc can be determined by the intensity of the light for a monochromatic light source, or by the constructive or destructive wavelength of the light for a white light source. A computer is programmed to receive data from the flying height tester and calculates the perceived flying height and angle of the head. With the rapid advance of disc drive technology, the flying height of many modem disc drives is less than 0.1 microns. Therefore, the accuracy of the flying height tester, and therefore its calibration, is an important concern.
Calibration of flying height testers has been accomplished through the use of a standard head whose characteristics are known. However, after repeated use, the reflective surface and flying characteristics of the head are altered by dust, oil and other foreign matter, altering the calibration of the standard. Calibration of flying height testers has also been accomplished through the use of a standard comprising a substrate having a reflective layer deposited thereon to represent the head and a transparent layer having a predetermined thickness deposited on the reflective layer. The standard is then placed in the flying height tester with the transparent layer spaced from the disc and monochromatic light is directed at the standard. A disadvantage of such a standard is that it uses a transparent material rather than air between the disc and the reflective layer. In addition, such a standard does not provide for the accurate determination of position along the length of the standard.